With respect to the art of orienting short fiber mixed in rubber or synthetic resin, known is a method for extruding rubber or synthetic resin mixed with short fiber, from an expansion die for extrusion molding having a sectionally annular mold opening. As for the expansion die for extrusion molding, for example the next one is included. The mold opening comprises an inlet opening, a middle opening (expanded portion), and an outlet opening. A sectional area of the outlet opening is formed larger than that of the inlet opening, a flow passage width of the inlet portion is smaller than that of the outlet portion, the flow passage width of the outlet portion is less than that of the middle opening, and the middle opening is provided with an expansion opening portion having an expanded flow passage width (e.g. Japanese Patent Publication No. H6-9847 (Pages 2 to 4, FIGS. 1 to 5).
When rubber or synthetic resin mixed with short fiber is extruded from this expansion die for extrusion molding, the orientation rate of the short fiber in the circumferential direction and the radius direction in the middle opening increases so that a cylindrical body is formed in which short fibers are oriented at a predetermined rate in the extrusion direction and in the circumferential direction and the radius direction, respectively.
The thus formed cylindrical body is cut open in the longitudinal direction to be formed into a sheet, and subsequently cut in the predetermined length, and it is used for bottom rubber of a transmission belt, buffer material, seal material, shoe sole, floor material, caterpillar cover material, etc. Since the sheet material mixed with short fibers has high strength and modulus in the orientation direction, formed sheet material (i.e. cylindrical body) has a different balance in orientation rate in each direction according to application. For example, in the case of the sheet material used for bottom rubber of the V belt, where lateral pressure resistance against load laterally received is required to prevent deformation, the cylindrical body having a high orientation rate in the circumferential direction is formed and this is cut open in the longitudinal direction to the predetermined length and cut in the width direction at predetermined intervals. Thus the sheet material with short fibers oriented in the width direction can be obtained by cutting open the cylindrical body. Therefore the sheet material can be more efficiently molded compared with the conventional calendar molding method where the cut sheet materials are connected at width direction end portions.
Meanwhile, when the rubber or synthetic resin mixed with short fibers is extruded from the above described expansion die having the mold opening, the short fibers are apt to be oriented in the circumferential direction. However, it is more apt to be oriented in the extrusion direction and in the radius direction depending on the shapes of the mold opening, and the orientation rate in the circumferential direction is lowered. Therefore, it is difficult in some cases to form the cylindrical body that satisfies the mechanical property in the circumferential direction.